Technical Field
The disclosure relates to a polymerizable polar compound, a liquid crystal composition, and a liquid crystal display element, and more specifically, to a polymerizable polar compound having both a perfluoroalkyl chain and an acryloyloxy group substituted with a polar group such as a hydroxyalkyl group, a liquid crystal composition containing the compound and having positive or negative dielectric anisotropy, and a liquid crystal display element including the composition or a cured material of a part thereof.
Description of Related Art
Liquid crystal display elements can be classified into modes such as phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), fringe field switching (FFS), and field-induced photo-reactive alignment (FPA) based on an operation mode of liquid crystal molecules. In addition, liquid crystal display elements can be classified as passive matrix (PM) and active matrix (AM) based on an element driving method. PMs are classified into static and multiplex matrixes and AMs are classified as thin film transistor (TFT) and metal insulator metal (MIM). In addition, TFTs can be classified as amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. Liquid crystal display elements can be classified into a reflective type that uses natural light, a transmissive type that uses a backlight, and a semi-transmissive type that uses both natural light and a backlight based on a light source.
A liquid crystal composition having a nematic phase has appropriate characteristics. It is possible to obtain an AM element having favorable characteristics by improving characteristics of this composition. The relationship between characteristics of the composition and characteristics of the AM element is summarized in the following Table 1.
TABLE 1Characteristics of compositions and AM elementsCharacteristicsCharacteristics ofNumberof compositionAM element1Wide temperature rangeWide temperature rangeof a nematic phasein which the elementcan be used2Low viscosity1)Short response time3Appropriate opticalLarge contrast ratioanisotropy4Large positive orLow threshold voltage,negative dielectriclow power consumption,anisotropyand large contrast ratio5Large specificHigh voltage holdingresistanceratio and largecontrast ratio6Stable with respectLong lifespanto ultraviolet radiationand heat7Large elastic constantLarge contrast ratio andshort response time1)The time taken for a composition to be inserted into a liquid crystal display element can be shortened
Characteristics of the composition will be further described based on commercially available AM elements. A temperature range of a nematic phase (a temperature range in which a nematic phase is exhibited) is related to a temperature range in which an element can be used. A preferable upper limit temperature of a nematic phase is about 70° C. or higher and a preferable lower limit temperature of a nematic phase is about −10° C. or lower.
The viscosity of the composition is related to a response time of the element. A short response time is preferable in order to display a moving image with elements. A shorter response time is desirable even if it is only 1 millisecond. Therefore, a lower viscosity of the composition is preferable and a lower viscosity at low temperatures is more preferable.
The optical anisotropy of the composition is related to a contrast ratio of the element. According to a mode of the element, large optical anisotropy or small optical anisotropy, that is, appropriate optical anisotropy is necessary. A product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the element is designed to maximize the contrast ratio. An appropriate product value depends on the type of operation mode. This value is about 0.45 μm in an element in a mode such as TN. This value is a range of about 0.30 μm to about 0.40 μm in an element in a VA mode and is a range of about 0.20 μm to about 0.30 μm in an element in an IPS mode or an FFS mode. In these cases, in an element having a small cell gap, a composition having large optical anisotropy is preferable.
Large dielectric anisotropy of the composition contributes to a low threshold voltage, low power consumption, and a large contrast ratio in the element. Therefore, large positive or negative dielectric anisotropy is preferable. A large specific resistance of the composition contributes to a high voltage holding ratio and a large contrast ratio in the element. Therefore, a composition having a large specific resistance not only at room temperature in an initial stage of use but also at a temperature close to an upper limit temperature of a nematic phase is preferable. A composition having a large specific resistance not only at room temperature but also at a temperature close to an upper limit temperature of a nematic phase after long term use is preferable.
The stability of the composition with respect to ultraviolet radiation and heat is related to a lifespan of the element. When this stability is higher, the lifespan of the element is longer. Such a characteristic is preferable for an AM element used in a liquid crystal projector and a liquid crystal television.
In a polymer sustained alignment (PSA) type liquid crystal display element, a liquid crystal composition containing a polymer is used. First, a composition to which a small amount of a polymerizable compound is added is inserted into the element. Here, a polymerizable compound having a plurality of polymerizable groups is generally used. Next, ultraviolet rays are emitted to the composition while a voltage is applied between substrates of the element. The polymerizable compound is polymerized and forms a polymer network structure in the composition. When this composition is used, since it is possible to control the alignment of liquid crystal molecules according to the polymer, a response time of the element is shortened and image burn is lessened. Such effects of the polymer can be expected for elements having modes such as TN, ECB, OCB, IPS, VA, FFS, and FPA.
In general purpose liquid crystal display elements, vertical alignment of liquid crystal molecules is achieved using a polyimide alignment film. On the other hand, for liquid crystal display elements having no alignment film, a mode in which a polar compound is added to a liquid crystal composition and liquid crystal molecules are aligned has been proposed. First, a composition in which a small amount of a polar compound and a small amount of a polymerizable compound are added is inserted into the element. As the polymerizable compound, a polymerizable compound having a plurality of polymerizable groups is generally used. Here, liquid crystal molecules are aligned according to an action of the polar compound. Next, ultraviolet rays are emitted to the composition while a voltage is applied between substrates of the element. Here, the polymerizable compound is polymerized and stabilizes the alignment of liquid crystal molecules. When this composition is used, since it is possible to control the alignment of liquid crystal molecules according to the polar compound and the polymer, a response time of the element is shortened and image burn is lessened. In addition, in elements having no alignment film, a process of forming an alignment film is unnecessary. Since there is no alignment film, a reduction in electrical resistance of the element due to an interaction between the alignment film and the composition does not occur. Such an effect caused by a combination of the polar compound and the polymer can be expected for elements having modes such as TN, ECB, OCB, IPS, VA, FFS, and FPA.
In liquid crystal display elements having no alignment film, as compounds to be added to a liquid crystal composition in order to align liquid crystal molecules, polar compounds having a perfluoroalkyl chain have been synthesized so far (Patent Document 1). For example, in Patent Document 1, a polar compound (S-1) having a perfluoroalkyl chain and a hydroxyl group is described.

[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2016-108310